1. Field of the Invention
This invention relates generally to dilatometers, and, more particularly, to dilatometers which permit precise temperature control and accurate dilation measurements of a test specimen within the range of about -65.degree. C. to about 150.degree. C. or higher.
2. Description of the Prior Art
Dilatometers are analytical instruments that respond to the linear thermal expansion or contraction of solids. Generally, these instruments employ a variable temperature electric furnace in which the test specimen is heated at a programmed rate to a desired elevated test temperature. The test specimen, which commonly is from about 10 to about 50 mm in length, is held within such furnace between a flat surface on a stationary object and an opposing flat surface on a movable object, such as a ceramic pushrod, that extends outside the furnace. Temperature induced changes in the length of the specimen are transmitted through the rod to a dilation sensor, which can be a mechanical, optical or electrical system for amplifying and measuring that change. These instruments are useful for measuring specimen dilation within the range of from ambient temperature to the maximum temperature of the furnace, which commonly is about 1000.degree. C. and often is as high as 1500.degree. C. or more.
Among the least complicated dilatometers in common use are those in which the push rod is coupled to a dial gauge and the dilation of a specimen is read directly from that gauge. Such dial gauge dilatometers are simple to use and inexpensive, but generally are suitable only for low to moderately elevated temperature applications that do not demand great precision.
U.S. Pat. No. 3,680,357 describes a far more precise type of dilatometer in which the dilation sensor is a linear variable differential transformer which translates specimen dilation into electrical signals that can readily be amplified and recorded. In such sensor, the core floats freely in the coil and each of these elements is separately supported at its ends by a pair of compound cantilevered flat springs. These springs permit independent and frictionless axial movement of the suspended element, but restrain radial or transverse movement. This independent and frictionless axial mobility of the core and coil facilitates calibration of the sensor and renders it extremely sensitive to minute changes in specimen length, thereby making possible exceptionally accurate measurements of thermally induced expansion or contraction.
When such dilatometer is used with a single pushrod, as shown in FIG. 1 of the aforementioned Patent, that pushrod commonly is coupled to and supported only by the core of the linear variable differential transformer and it extends into the open end of a ceramic specimen tube, where it abuts a specimen that is held between a flat ground surface at the end of the pushrod and a similar flat ground surface on the interior of the other closed end of the specimen tube. An opening commonly is provided in the wall of the specimen tube adjacent to its closed end to facilitate specimen changes. The closed end of the specimen tube is inserted into a variable temperature furnace, which, for many applications, is a conventional electric tube furnace.
For measurements of the differential thermal expansion of two specimens, separate closely spaced pushrods may be coupled to the core and coil of the linear variable differential transformer and the equally closely spaced specimens are held abreast within a single tubular specimen tube that is inserted in a similar electric tube furnace, as shown by U.S. Pat. No. 3,898,836.
While the typical dilatometers described above generally are used for dilation measurements at elevated temperatures, they also can be used for measurements at temperatures below ambient by substituting a cryostat for the electric furnace. Exemplary of such low temperature dilatometers is the instrument described in U.S. Pat. No. 4,351,615, in which the specimen, enclosed in a protective fused silica tube, is immersed in a cryostat containing liquid helium. The temperature within the cryostat is raised for dilation measurements by an electric heating sleeve that surrounds the protective tube. In order to avoid a magnetic field influence on the specimen, the wire in the heating sleeve is wound "two wires in hand" after bending the wire in a loop thus forming two coils having compensating fields.
While this cryostat enables the user to achieve initial specimen temperatures which approach the -268.9.degree. C. boiling point of helium, such extreme low temperatures are not essential for the great majority of dilation studies, which require minimum and maximum temperatures similar to those normally encountered by the material being tested. For example, military specifications for printed circuit boards and electronic devices on those boards call for an operating free-air temperature range of -55.degree. to 125.degree. C. and a storage temperature range of -65.degree. to 150.degree. C. Even these extreme ranges, which rarely, if ever, are actually experienced, are considerably higher than the boiling point of helium. For most materials destined for civilian applications, the temperature range of greatest interest is defined by the expected winter low and summer high, which, in a temperate climate, rarely is outside the range of from about -30.degree. to about 45.degree. C. Measurements at these far higher temperatures are delayed by the fact that all or essentially all of the liquid helium must be vaporized and vented from the cryostat before there can be any significant rise in the temperature of the specimen. This necessitates lengthy operation of the heating sleeve and extremely precise control in order to avoid building up excessive internal pressure while the liquid helium is being boiled off and also to avoid temperature surges thereafter.
Another disadvantage of the low temperature dilatometer of U.S. Pat. No. 4,351,615, as well as similar instruments employing liquid nitrogen cooled cryostats, is the fact that a conventional length specimen dilates so little over a narrow temperature range, such as that experienced in a temperate climate, that otherwise small sensor, amplifier and recorder errors are exaggerated. One may only partially compensate for this by lengthening the specimen, as any increase in the mass of the specimen increases the possibility of temperature differentials and a decrease in the thickness of the lengthened specimen may make it insufficiently rigid to withstand the compressive force imposed by the pushrod.